In general this invention relates to actuators and, in particular, electromagnetic actuators operable for use in progressively driving a movable element along a given path.
In autofocusing systems a rangefinder device derives a signal that is a function of the subject-to-camera range. Such a signal drives a movable lens element of a lens assembly to a preselected focal position corresponding to the subject range. Examples of the foregoing systems are described generally in U.S. Pat. Nos. 3,522,764; 4,149,792; 4,153,903; and 4,199,244. Broadly, these patents describe analog components for driving the lens. For instance, the output of a rangefinder displaces a lens element through mechanical devices, such as a cam system, gear train, or an electrical device including a non-linear potentiometer for driving a conventional electric motor. One potential problem with analog drive components is that they do not precisely displace the lens element to the desired focal position.
One known approach for improving upon analog drive devices, in terms of accurately positioning a movable element, is with an electromagnetic stepper motor. These motors precisely increment displacement of an output member to precise positions. For instance, commonly assigned U.S. Pat. No. 3,882,522 discloses use of a rotary stepper motor for accurately controlling displacement of a shutter mechanism. However, stepper motors of the rotary type are not necessarily satisfactory under all circumstances. For instance, rotary stepper motors tend to be unsuitable for driving a lens element in an autofocusing system because they are relatively large and require relatively substantial amounts of power.
Linear motors, such as the kind described in commonly-assigned U.S. Pat. No. 4,264,154, have been proposed to rotationally incrementally drive an optical element for varying the transmissivity characteristics of sunglasses in accordance with ambient light. While such a linear motor serves admirably in the above circumstances, its potential use for rotationally displacing a lens element in an autofocusing system is somewhat limited. This is because it does not transmit motion during its return stroke. Thus, it lacks the speed necessary for displacing the lens element within the required time parameters of an autofocusing system.